Important Dates

• Assigned: Tuesday, April 9, 2002
• Due: Monday, April 22, 2002 at 11:59PM

Introduction

In this lab you will be writing a dynamic storage allocator for C programs, i.e., your own version of the malloc(), free() and realloc() routines. You are encouraged to explore the design space creatively and implement an allocator that is correct, efficient and fast.

Logistics

You may work in a group of up to two people. Any clarifications and revisions to the assignment will be posted on the course Web page.

Hand Out Instructions

The files for this assignment can be retrieved from

/afs/cs/academic/class/15213-s02/labs/L6/L6.tar

Once you've copied this file into a (protected) directory, run the command {\tt tar xvf L6.tar}. Fill in your team information in the structure at the beginning of the file {\tt mm.c}. When you have completed the lab, you will hand in only one file ({\tt mm.c}), which contains your solution.

How to Work on the Lab

Your dynamic storage allocator will consist of the following four functions, which are declared in mm.h and defined in mm.c.

    int   mm_init(void);
    void *mm_malloc(size_t size);
    void  mm_free(void *ptr);
    void *mm_realloc(void *ptr, size_t size);

The mm.c file we have given you implements the simplest but still functionally correct malloc package that we could think of. Using this as a starting place, modify these functions (and possibly define other private static functions), so that they obey the following semantics:

• mm_init(): Before calling mm_malloc() mm_realloc() or mm_free(), the application program (i.e., the trace-driven test harness that you will use to evaluate your implementation) calls mm_init() to perform any necessary initializations, such as allocating the initial heap area. The return value should be -1 if there was a problem in performing the initialization, 0 otherwise.
  
  
• mm_malloc(): The mm_malloc() routine returns a pointer to an allocated block payload of at least {\tt size} bytes. The entire allocated block should lie within the heap region and should not overlap with any other allocated chunk.

  We will comparing your implementation to the version of malloc() supplied in the standard C library ({\tt libc}). Since the libc malloc() always returns payload pointers that are aligned to 8 bytes, your malloc implementation should do likewise and always return 8-byte aligned pointers.
  
  

• mm_free(): The mm_free() routine frees the block pointed to by ptr. It returns nothing. This routine is only guaranteed to work when the passed pointer (ptr) was returned by an earlier call to mm_malloc() or mm_realloc() and has not yet been freed.
  
  
• mm_realloc(): The mm_realloc() routine returns a pointer to an allocated region of at least size bytes with the following constraints:
  • if ptr is NULL, the call is equivalent to mm_malloc(size);
    
    
  • if size is equal to zero, the call is equivalent to mm_free(ptr);
    
    
  • if ptr is not NULL, it must have been returned by an earlier call to mm_malloc() or mm_realloc(). The call to mm_realloc() changes the size of the memory block pointed to by ptr (the old block) to size bytes and returns the address of the new block. Notice that the address of the new block might be the same as the old block, or it might be different, depending on your implementation, the amount of internal fragmentation in the old block, and the size of the realloc() request.

    The contents of the new block are the same as those of the old ptr block, up to the minimum of the old and new sizes. Everything else is uninitialized. For example, if the old block is 8 bytes and the new block is 12 bytes, then the first 8 bytes of the new block are identical to the first 8 bytes of the old block and the last 4 bytes are uninitialized. Similarly, if the old block is 8 bytes and the new block is 4 bytes, then the contents of the new block are identical to the first 4 bytes of the old block.

These semantics match the the semantics of the corresponding Linux malloc(), realloc(), and free() routines. Type man malloc at the shell for complete documentation.

Heap Consistency Checker

Dynamic memory allocators are notoriously tricky beasts to program correctly and efficiently. They are difficult to program correctly because they involve a lot of (void *) pointer references. You will find it very helpful to write a heap checker that scans the heap and checks it for consistency.

Some examples of what a heap checker might check are:

• Is every block in the free list marked as free?
  
  
• Are there any contiguous free blocks that somehow escaped coalescing?
  
  
• Is every free block actually in the free list?
  
  
• Do the pointers in the free list point to valid free blocks?
  
  
• Do any allocated blocks overlap?
  
  
• Do the pointers in a heap block point to valid heap addresses?

Your heap checker will consist of the function int mm_check(void) in mm.c. It will check any invariants or consistency conditions you consider prudent. It returns a nonzero value if and only if your heap is consistent. You are not limited to the listed suggestions nor are you required to check all of them. You are encouraged to print out error messages when mm_check() fails.

This consistency checker is for your own debugging during development. When you submit mm.c, make sure to remove any calls to mm_check() as they will slow down your throughput. Style points will be given for your mm_check(). Make sure to put in comments and document what you are checking.

Support routines

The memlib.c package simulates the memory system for your dynamic memory allocator. You can invoke the following functions in memlib.c:

• void *mem_sbrk(int incr): Expands the heap by incr bytes, where incr is a positive non-zero integer and returns a generic pointer to the first byte of the newly allocated heap area. The semantics are identical to the Unix sbrk() function, except that mem_sbrk() accepts only a positive non-zero integer argument.
  
  
• void *mem_heap_lo(void): Returns a generic pointer to the first byte in the heap.
  
  
• void *mem_heap_hi(void): Returns a generic pointer to the last byte in the heap.
  
  
• size_t mem_heapsize(void): Returns the current size of the heap in bytes.
  
  
• size_t mem_pagesize(void): Returns the system's page size in bytes (4K on Linux systems).

The Trace-Driven Test Harness

The mtest.c program in the L6.tar distribution tests your mm.c package for correctness, space utilization, and throughput. The test harness is controlled by a set of trace files that are included in the L6.tar distribution. Each trace file contains a sequence of allocate, reallocate, and free directions that instruct the test harness to call your mm_malloc(), mm_realloc(), and mm_free() routines in some sequence. The test harness and the trace files are the same ones we will use when we grade your handin mm.c file.

The test harness accepts the following command line arguments:

• -f <tracefile>: Use one particular tracefile for testing instead of the default set.
• -h: Print a summary of the command line arguments.
  
  
• -l: Run libc malloc in addition to the student's malloc package.
  
  
• -v: Verbose output. Print a performance breakdown for each tracefile.
  
  
• -V: Prints additional diagnostic information as each trace file is processed. Useful during debugging for determining which trace file is causing your malloc package to fail.

Programming Rules

• You are not allowed to change any of the interfaces in mm.c.
  
  
• You should not invoke any memory-management related library calls or system calls. This excludes the use of malloc(), calloc(), free(), realloc(), sbrk(), brk() or any variants of these calls in your code.
  
  
• You are not allowed to define any global or static compound data structures such as arrays, structs, trees, or lists in your mm.c program. However, you are allowed to declare global scalar variables such as integers, floats, and pointers in mm.c.
  
  
• For consistency with the libc malloc package, your allocator must always return pointers that are aligned to 8-byte boundaries. The test harness will enforce this for you.

Evaluation

You will receive zero points if you break any of the rules or your code is buggy. Otherwise, your grade will be calculated as follows:

• Correctness (20 points). You will receive full points if your solution passes the correctness tests performed by the test harness (mtest}). You will receive partial credit for correct implementations of malloc() and free() (i.e., you pass the first 9 trace files).
  
  
• Performance (35 points). Two performance metrics will be used to evaluate your solution:
  • Space utilization: The peak ratio between the aggregate amount of memory used by the test harness (i.e., allocated via mm_malloc() or mm_realloc() but not yet freed via mm_free() and the size of the heap used by your allocator. The optimal ratio equals to 1. You should find good policies to minimize fragmentation in order to make this ratio as close as possible to the optimal.
    
    
  • Throughput: The average number of operations completed per second.

  The test harness (mtest) summarizes the performance of your allocator by computing a performance index, P, which is a weighted sum of the space utilization and throughput

  P = wU + (1-w)min(1, T/T_libct)

  where U is your space utilization, T is your throughput, and T_libc is the estimated throughput of libc malloc on your system on the default traces (600 Kops/sec). The index favors space utilization over throughput, with a default of $w = 0.6$.

  Observing that both memory space and CPU cycles are expensive system resources, we adopt this formula to encourage balanced optimization of both. Ideally, the performance index will reach P = w + (1-w) = 1 or 100%. Since each metric will contribute at most w and 1-w to the performance index, respectively, you should not go to extremes to optimize either the memory utilization or the throughput only. To receive a good score, you must achieve a balance between utilization and throughput.

  To get full credit for this part of the lab, you will need to achieve a performance index of 95%. Your score will be calculated by adding .05 to your performance index and multiplying by 35 points. You cannot get more than 35 points for this part of the lab, though.
  
  

• Style (10 points). Your code should be readable and well commented. Define macros or subroutines where necessary to make the code more understandable. 5 points will be given for a well written and documented mm_check() and 5 points will be given for the style of the rest of your code.

You will handin your mm.c file via a web interface. See the lab webpage for details on how to do this.

You may submit your solution for testing as many times as you wish up until the due date. The web page will list both your best scoring submission and your most recent submission.

When you are satisfied with your solution, then you can officially hand it in. Only the last version you submit will be graded.

When testing your files locally, make sure to use one of the fish machines. This will insure that the grade you get from mtest is representative of the grade you will receive when you submit your solution.

• Use the "mtest -f" option. During initial development, using tiny trace files will simplify debugging and testing. We have included two such trace files (short1,2-bal.rep) that you can use for initial debugging.
  
  
• Use the "mtest -v" and "-V" options. The "-v" option will give you a detailed summary for each trace file. The "-V" will also indicate when each trace file is read, which will help you isolate errors.
  
  
• Compile with "gcc -g" and use a debugger. A debugger will help you isolate and identify out of bounds memory references.
  
  
• Understand every line of the malloc implementation in the textbook. The textbook has a detailed example of a simple allocator based on an implicit free list. Use this is a point of departure. Don't start working on your allocator until you understand everything about the simple implicit list allocator.
  
  
• Encapsulate your pointer arithmetic in C preprocessor macros. Pointer arithmetic in memory managers is confusing and error-prone because of all the casting that is necessary. You can reduce the complexity significantly by writing macros for your pointer operations. See the text for examples.
  
  
• Do your implementation in stages. The first 9 traces contain requests to malloc() and free(). The last 2 traces contain requests for realloc(), malloc(), and free(). We recommend that you start by getting your malloc() and free() routines working correctly and efficiently on the first 9 traces. Only then should you turn your attention to the realloc() implementation. For starters, build realloc() on top of your existing malloc() and free() implementations. But to get really good performance, you will need to build a stand-alone realloc().
  
  
• Use a profiler. You may find the gprof tool helpful for optimizing performance.
  
  
• Start early! It is possible to write an efficient malloc package with a few pages of code. However, we can guarantee that it will be some of the most sophisticated code you have written so far in your career. So start early, and good luck!